At the end of this lecture students should be able to; Define the C standard functions for managing file input output. Apply taught concepts for writing programs.

1. COM1407
Computer Programming
Lecture 14
File Processing
K.A.S.H. Kulathilake
B.Sc. (Hons) IT, MCS , M.Phil., SEDA(UK)
Rajarata University of Sri Lanka
Department of Physical Sciences
1

2. Objectives
• At the end of this lecture students should be able
to;
▫ Define the C standard functions for managing file
input output.
▫ Apply taught concepts for writing programs.
2

3. Introduction
• The file is the basic unit of storage for many
operating systems.
• Although C does not have any built-in method
for performing file I/O, the C standard library
contains a very rich set of I/O functions
providing and efficient, powerful and flexible
approach.
• Those functions can read or write any type of
data to files.
3

4. What is a File?
• In C, a file can refer to a disk file and every disk file
has a name.
• Filenames are stored as strings, just like other text
data.
• In DOS and Windows 3.X, a complete filename
consists of a name that has from one to eight
characters, optionally followed by a period and an
extension that has from one to three characters.
• In contrast, the Windows 95 and Windows NT OS,
as well as most UNIX systems, permit filenames
upto 256 characters long.
4

5. What is a File? (Cont…)
• A file name in C program also can contain path
information.
• The path specifies the drive and or directory where
the file is located.
▫ C:DATAlist.txt  this is a good programming
practice to specify path information. is used to
separate the directory names in the path.
• If you specify a filename without a path, it will be
assumed that the file is located at whatever location
the OS currently designates as the default.
char* filename = “C:DATAlist.txt”;
5

6. What is a Stream?
• The data flow you transfer from your program to a file,
or vice versa, is called a stream which is a series of bytes.
• Hence, a stream is a sequence of bytes travelling from a
source to a destination over a communication medium.
• Streams can either be input or output.
• The input stream is used for reading while the output
stream is used for writing.
• Unlike a file, which targets a specific I/O device, a
stream is device independent.
• You can perform I/O operations to any type of file by
simply associating a stream to the file.
6

7. What is a Stream? (Cont…)
• There are two formats of streams.
• Text streams
▫ Consists of sequence of characters.
▫ Depending on the system, each line of characters
in a text stream may be terminated by a newline
character.
▫ Text streams are used for textual data, which has a
consistent appearance from one environment to
another, or from one machine to another.
7

8. What is a Stream? (Cont…)
• Binary stream
▫ It consists of series of bytes, e.g., the contents of
an executable program file.
▫ Binary streams are primarily used for nentextual
data, where the exact contents of the data are
maintained without regard to appearance.
8

9. Buffered I/O
• In C, a memory area that is temporarily used to store
data before it is sent to its destination is called a buffer.
• With the help of buffers, the operating system can
improve efficiency by reducing the number of accesses to
I/O devices (that is files).
• Access to a disk or other I/O device is generally much
slower than direct access to memory.
• Several I/O operations can be performed on a buffer that
represents the I/O file, instead of the file itself.
• e.g. if several write operations are sent to a buffer, they
are kept in memory until it is time to save, or commit,
the new data to the actual device.
9

10. Pointers of FILE
• The FILE structure is the file control structure
defined in the header file stdio.h.
• A pointer of type FILE is called a file pointer,
which references a disk file.
• A file pointer is used by a stream to conduct the
operation of the I/O function.
• e.g. following defines a file pointer called fptr;
FILE *fptr;
10

11. Opening a File
• fopen() opens a file and associates a stream with the
opened file.
#include <stdio.h>
FILE *fopen( const char* filename,
const char *mode);
• Here, filename is a char pointer that references a string
containing a filename.
• Mode points to another string that specifies the way to
open the file.
• The fopen() function returns a pointer of type FILE in a
success scenario otherwise null pointer is returned.
11

12. Opening a File (Cont…)
12
Mode Description
r Opens an existing text file for reading purpose.
w Opens a text file for writing. If it does not exist, then a new file is created. Here
your program will start writing content from the beginning of the file.
a Opens a text file for writing in appending mode. If it does not exist, then a new
file is created. Here your program will start appending content in the existing
file content.
r+ Opens a text file for both reading and writing.
w+ Opens a text file for both reading and writing. It first truncates the file to zero
length if it exists, otherwise creates a file if it does not exist.
a+ Opens a text file for both reading and writing. It creates the file if it does not
exist. The reading will start from the beginning but writing can only be
appended.

13. Opening a File (Cont…)
13
Mode Description
rb Opens an existing binary file for reading
wb Creates a binary file for writing
ab Opens an existing binary file for appending
r+b or
rb+
Opens an existing binary file for reading or writing
w+b or
wb+
Creates a binary file for reading or writing
a+b
ab+
Opens or creates a binary file for appending

14. Opening a File (Cont…)
#include <stdio.h>
int main ()
{
FILE *fptr;
fptr = fopen("test.txt","r");
if (fptr == NULL )
{
printf("Cannot open file.n");
exit (1);
}
else
{
printf("File opens successfully.n");
}
return 0;
}
14

15. Closing a File
• After a disk file is read, written or appended with some new data,
you have to disassociate the file from a specified stream by calling
the fclose() function.
#include <stdio.h>
int fclose ( FILE *stream );
• Here stream is a file pointer that is associated with a stream to the
opened file.
• If fclose() closes a file successfully, it returns 0, otherwise it returns
EOF.
• Normally, fclose() fails only when the disk is removed before the
function is called or there is no more space left on the disk.
• The fclose() flushes data left in the buffer to ensure that no data will
be lost before it disassociates a specified stream with the opened file.
15

16. Closing a File (Cont…)
#include <stdio.h>
int main ()
{
FILE *fptr;
fptr = fopen("test.txt","r");
if (fptr == NULL )
{
printf("Cannot open file.n");
exit (1);
}
16

17. Closing a File (Cont…)
else
{
int status = 0;
printf("File opens successfully.n");
status = fclose(fptr);
if (status == 0 )
printf("Status: %i - File closed
successfully.n", status);
else
printf("Error in file closing.n");
}
return 0;
}
17

18. I/O Operations
• In C, you can perform I/O operations in the
following ways;
▫ Read or write one character at a time.
▫ Read or write one line of text at a time.
▫ Read or write one block of characters at a time.
18

19. Read One Character at a Time
• fgetc() fucntion is used to read one character at a time.
#include <stdio.h>
int fgetc( FILE *stream);
• Here stream is the file pointer associated with a stream.
• The fgetc() fetches the next character from the stream.
• The function then returns value of an int that is
converted from the character.
• EOF is returned if fgetc() encounters the end of file or if
there is an error.
19

20. Read One Character at a Time (Cont…)
#include <stdio.h>
int main ()
{
FILE *fptr;
char fileName [] = "test.txt";
if ( (fptr = fopen (fileName, "r")) == NULL){
printf("Cannot open file.n");
exit (1);
}
else{
int c;
while ( (c = fgetc(fptr))!= EOF){
putchar (c);
}
fclose(fptr);
}
return 0;
}
20

21. Write One Character at a Time
• fputc() fucntion is used to write one character at a time.
#include <stdio.h>
int fputc (int c, FILE *stream);
• Here c is an int value that represents a character.
• In fact, the int value is converted to an unsigned char
before being output.
• Stream is the file pointer associated with a stream.
• The fputc() returns the character written if the function
is successful; otherwise, it returns EOF.
• After a character is written, the fputc() advances the
associated file pointer.
21

22. Write One Character at a Time (Cont…)
#include <stdio.h>
int main ()
{
FILE *fp;
int ch;
fp = fopen("writec.txt", "w+");
for( ch = 33 ; ch <= 100; ch++ )
{
fputc(ch, fp);
}
fclose(fp);
return(0);
}
22

23. Write One Character at a Time (Cont…)
#include <stdio.h>
#include <string.h>
int main ()
{
FILE *fptr;
char fileName [] = "writec.txt";
char text[100] = "This content to be written
in to a file";
int lstr = strlen( text );
if ( (fptr = fopen (fileName, "w+")) == NULL)
{
printf("Cannot open file.n");
exit (1);
}
23

24. Write One Character at a Time (Cont…)
else
{
int i;
for(i = 0 ; i <= lstr ; i++)
fputc (text[i],fptr);
printf("End");
fclose(fptr);
}
return 0;
}
24

25. Activity
#include <stdio.h>
void charReadWrite( FILE*, FILE*);
int main ()
{
FILE *fptr1, *fptr2;
char fileName1 [] = "writec.txt";
char fileName2 [] = "test.txt";
if ( (fptr1 = fopen (fileName1, "w+"))
== NULL){
printf("Cannot open %s file.n", fileName1);
exit (1);
}
else{
25

26. Activity
if ( (fptr2 = fopen (fileName2, "r"))
== NULL){
printf("Cannot open %s file.n",
fileName2);
exit (1);
}
else{
charReadWrite (fptr2, fptr1);
fclose (fptr1);
fclose (fptr2);
}
}
return 0;
}
26

27. Activity
void charReadWrite( FILE *fin, FILE *fout)
{
int c;
while ((c = fgetc(fin)) != EOF)
{
fputc(c, fout);
putchar(c);
}
}
27

28. Read One Line of Text at a Time
• fgets() function is used to read one line at a time.
#include <stdio.h>
char *fgets (char *s, int n, FILE *stream);
• Here s refers a character array that us used to store characters
read from the opened file pointed to by the file pointer
stream.
• n specifies the maximum number of array elements.
• If it is successful, the fgets() function returns a char pointer s.
• If EOF is encountered, the fgets() function returns a null
pointer and leaves the array untouched.
• If an error occurs, the function returns a null pointer, and the
contents of the array are unknown.
28

29. Read One Line of Text at a Time
(Cont…)
#include <stdio.h>
int main ()
{
FILE *fptr;
char fileName [] = "test.txt";
if ( (fptr = fopen (fileName, "r")) == NULL){
printf("Cannot open file.n");
exit (1);
}
else{
char buff [80];
while ( fgets(buff, 80, fptr)!= NULL){
printf("%s", buff);
}
fclose(fptr);
}
return 0;
}
29

30. Read One Line of Text at a Time
(Cont…)
• The fgets() can read up to n-1 characters, and
can append a null character after the last
character fetched, until a newline or and EOF is
encountered.
• Note that the new line is encountered during the
read operation, the fgets() function includes the
newline in the array.
• This is different from what the gets() does.
• The gets() just replaces the newline character
with a null character.
30

31. Write One Line of Text at a Time
• fputs() function is used to write one line at a time.
#include <stdio.h>
char *fputs (const char *s, FILE *stream);
• Here s points to the array that contains the characters to
be written to a file associated with the file pointer
stream.
• The const modifier indicates that the content of the array
pointed to by s cannot be changed by the fputs().
• If it is fails, the fputs() function returns a nonzero value;
otherwise, it returns zero.
31

32. Write One Line of Text at a Time
(Cont…)
• Note that the character array must include a null
character at the end of the string as the
terminator to the fputs().
• Also, unlike the puts(), the fputs() does not
insert a newline character to the string written to
the file.
32

33. Write One Line of Text at a Time
(Cont…)
#include <stdio.h>
#include <string.h>
int main ()
{
FILE *fptr;
char fileName [] = "writec.txt";
char text[100] = "This content to be written in to a filen";
int lstr = strlen( text );
if ( (fptr = fopen (fileName, "w+")) == NULL){
printf("Cannot open file.n");
exit (1);
}
else{
int i=0;
for(;i<10;i++)
fputs(text, fptr);
fclose(fptr);
}
return 0;
}
33

34. Activity
#include <stdio.h>
void lineReadWrite( FILE*, FILE*);
const int MAX_LEN = 81;
int main ()
{
FILE *fptr1, *fptr2;
char fileName1 [] = "writec.txt";
char fileName2 [] = "test.txt";
if ( (fptr1 = fopen (fileName1, "w+")) == NULL)
{
printf("Cannot open %s file.n", fileName1);
}
34

35. Activity
else
{
if ( (fptr2 = fopen (fileName2, "r")) == NULL)
{
printf("Cannot open %s file.n", fileName2);
}
else
{
lineReadWrite (fptr2, fptr1);
fclose (fptr1);
fclose (fptr2);
}
}
return 0;
}
35

36. Activity
void lineReadWrite( FILE *fin, FILE *fout)
{
char buff[100];
while ( fgets(buff, MAX_LEN, fin) != NULL)
{
fputs(buff, fout);
printf( "%s", buff);
}
}
36

37. Read One Block of Text at a Time
• fread() is used to reading one block of text at a time.
#include <stdio.h>
size_t fread ( void *ptr, size_t size, size_t n,
FILE *stream);
• Here ptr is a pointer to an array in which the data is stored.
• Size indicates the size of each array element.
• N specifies the number of elements to read.
• Stream is a file pointer that is associated with the opened file
for reading.
• size_t is a integral type of defined in the header file stdio.h.
• The fread() returns the number of elements actually read.
37

38. Read One Block of Text at a Time
(Cont…)
• The number of elements read by the fread()
function should be equal to the value specified
by the third argument to the function, unless an
error occurs or and EOF is encountered.
• The fread() returns the number of elements that
are actually read during the attempt, if an error
occurs or an EOF is encountered.
38

39. Write One Block of Text at a Time
• fwrite() is used to write one block of text at a time.
#include <stdio.h>
size_t fwrite( const void *ptr, size_t size,
size_t n,FILE *stream);
• Here ptr references the array that contains the data to be written to
an opened file pointed to by the file pointer stream.
• size indicates the size of each element in the array.
• n specifies the number of elements to be written.
• The fwrite() function returns the number of elements actually
written.
• If no error has occurred, the value written by fwrite() should equal
the third argument in the function.
• The return value may be less than the specified value in an error
occurs.
39

40. Write One Block of Text at a Time
(Cont…)
• Note that it is programmers’ responsibility to ensure
that the array is large enough to hold data for either
the fread() or the fwrite().
• In C, a function called feof() can be used to
determine when the end of a file is encountered.
#include <stdio.h>
int feof(FILE *stream);
• The feof() returns 0 if the end of the file has not
been reached; otherwise, it returns the non zero
integer.
40

41. Write One Block of Text at a Time
(Cont…)
#include <stdio.h>
#include <string.h>
int main()
{
FILE *fp;
char c[] = "This is about fread() and fwrite()";
char buffer[100];
/* Open file for both reading and writing */
fp = fopen("file.txt", "w+");
/* Write data to the file */
fwrite(c, strlen(c) + 1, 1, fp);
41

42. Write One Block of Text at a Time
(Cont…)
/* Seek to the beginning of the file */
fseek(fp, 0, SEEK_SET);
/* Read and display data */
fread(buffer, strlen(c)+1, 1, fp);
printf("%sn", buffer);
fclose(fp);
return(0);
}
42

43. Activity
#include <stdio.h>
void blockReadWrite( FILE*, FILE*);
const int MAX_LEN = 81;
int main ()
{
FILE *fptr1, *fptr2;
char fileName1 [] = "writec.txt";
char fileName2 [] = "test.txt";
if ( (fptr1 = fopen (fileName1, "w+")) == NULL)
{
printf("Cannot open %s file.n", fileName1);
}
43

44. Activity
else
{
if ( (fptr2 = fopen (fileName2, "r")) == NULL)
{
printf("Cannot open %s file.n", fileName2);
}
else
{
blockReadWrite (fptr2, fptr1);
fclose (fptr1);
fclose (fptr2);
}
}
return 0;
}
44

45. Activity
void blockReadWrite( FILE *fin, FILE *fout)
{
int num;
char buff[100];
while (!feof(fin))
{
num = fread(buff, sizeof(char), MAX_LEN, fin);
buff[num * sizeof(char)] = '0';
printf( "%s", buff);
fwrite(buff, sizeof(char), num,fout);
}
}
45

46. Objective Re-cap
• Now you should be able to:
▫ Define the C standard functions for managing file
input output.
▫ Apply taught concepts for writing programs.
46

47. References
• Chapter 16 - Programming in C, 3rd Edition,
Stephen G. Kochan
47

48. End of COM1407
Next: Data Structures & Algorithms, Compute Graphics &
Image Processing
48